A scientist at the forefront of cloning has detailed the potential benefits to medicine - and the huge hurdles experts must cross to reap them.

Professor Ian Wilmut, from the Roslin Institute, was one of the team which produced Dolly the sheep, the first mammal to be cloned from an adult cell.

In a lecture to the Royal Society of Medicine, he said that a scientist would have to be "ill" to want to apply the present techniques to humans.

But he said sufferers from a variety of conditions might eventually benefit from the methods his team have developed.

Cloning involves fusing genetic material from a separate adult or embryo cell into an egg, which then develops.

Cloning involves highly delicate techniques

However, the failure rate is enormous - on average, only just over 1% of attempts actually results in a live birth.

It took more than 250 attempts before Dolly the sheep was born.

And there are some animals which, unlike sheep and cattle, scientists have never managed to clone, despite repeated attempts.

These include primates, which suggests that human cloning may be nigh-on impossible using current methods.

However, Professor Roslin has already been approached by people asking whether he can help them clone a human.

He considers that the likely failure of attempts make it highly unethical to proceed to human experiments until the techniques have a higher success rate in animals.

'It is appalling'

He told the meeting: "It is quite obvious to me that it is appalling for anyone to suggest using this on a woman at the present time.

"I think you have to be ill to use this on humans."

Could cloned human babies happen one day?

In fact, he thinks that he would disapprove of any use of human cloning techniques to actually create a new person - for example, to assist in infertility, or replace a lost relative.

However, in the future, he sees genetic techniques such as these as a possible source of hope to people with certain illnesses.

"It could be helpful to treat conditions associated with damage to cells which don't repair themselves - there isn't currently an effective treatment for any of them."

Diseases in this category include Parkinson's, diabetes, liver damage, arthritis and macular degeneration - which can lead to blindness.

In each case, doctors might be able to use cloned stem cells.

This technique would involve taking cells from the disease sufferer and using the genetic code inside to clone an embryo.

This would contain stem cells, which can, depending on their environment, progress to become hundreds of different types of cell, each with a different function, such as brain neuron cells, or special pancreas cells which diabetics lack.

Scientists want to clone embryos for research

The stem cells, in theory, can be grown "in vitro", ensuring a ready supply.

They can be then inserted at the disease site, and hopefully replace the lost cells.

In Parkinson's, the patient lacks cells which produce a brain chemical called dopamine. If these are replaced, then in theory the symptoms and decline could be halted.

Aborted foetuses

At present, stem cells are harvested from aborted foetuses to produce a treatment - many foetuses are needed to make even one treatment.

The creation of embryos for medicine is a controversial ethical issue - the Government's chief medical officer in England, Sir Liam Donaldson, is heading an expert committee which will decide whether it will be allowed in the UK.

Professor Wilmut said: "I, personally, would be willing to think about doing this for this purpose."

There are other avenues in which genetic advances could produce viable treatments.

The Roslin Institute has already made genetic modifications to sheep which make them produce a protein in their milk that helps clot human blood.

This could eventually be of use to haemophiliacs, who lack the protein - and can suffer life-threatening bleeding if even slightly injured.

And Professor Wilmut also mentioned the genetic modification of pig organs so they can be used in human transplantation - another ethical minefield.

He said he shared some of Prince Charles' concerns about "potential misuse" of the techniques, but said that overall, the research had a great deal to contribute.

"What we argue for is very ambitious research - until we do this, we have no idea what we will find."